1. Technical Field
The present exemplary embodiments relate to a MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to perform MRA (magnetic resonance angiography) for obtaining a blood flow image with using blood flow information as a trigger.
2. Related Art
Magnetic Resonance Imaging is an imaging method which excites nuclear spins of an object set in a static magnetic field with an RF signal having the Larmor frequency and reconstructs an image based on NMR signals generated due to the excitation.
In the field of magnetic resonance imaging, as a method of obtaining an image of a blood flow, MRA is known. MRI that does not use a contrast medium is referred to as non-contrast MRA. As non-contrast MRA, an FBI (fresh blood imaging) method performs ECG (electro cardiogram) synchronization to capture a pumping blood flow ejected from the heart, thereby satisfactorily representing a blood vessel (for example, refer to JP-A No. 2000-5144). The FBI method is an imaging method for acquiring an angio image as a transverse relaxation (T2) weighted image by a three dimensional scan with a SE (spin echo) sequence.
Further, ECG-prep as a related technology used with the FBI method is devised to measure an appropriate delay time for ECG synchronization (see, for example, U.S. Pat. No. 6,144,201). ECG-prep is a technique that performs an ECG-prep scan as a preparation scan to decide upon an appropriate delay time for ECG synchronization for imaging an angio image satisfactorily prior to an FBI scan for diagnostic imaging and subsequently the FBI scan is performed with the ECG delay time decided upon by the ECG-prep scan. The ECG-prep scan is a pre-scan to obtain plural angio images at mutually different time phases by acquiring data while gradually changing delay times from a trigger R wave of an ECG. By selecting an angiogram, corresponding to a time phase at which brightness in a blood vessel part is higher, from the plural angiograms obtained by the ECG-prep scan, an ECG delay time for the FBI scan can be determined.
Meanwhile, a PC (phase contrast) MRA method is known as another method for non-contrast-enhanced MRA (see, for example, Japanese Publication of Patent Application No. 63-230157). The PC MRA method is also called a PS (phase shift) MRA method and an imaging method to generate an image of a blood flow from phase information of spins. More specifically, in the PC MRA method, only moving spins are imaged selectively using a phenomenon that phases of static spins do not change before and after application of a bipolar gradient magnetic field, in contrast, phases of moving spins in blood flow shift before and after application of the bipolar gradient magnetic field in case where the bipolar gradient magnetic field is applied. A gap in phase of spins generated after applying a gradient magnetic field depends on an intensity and an application period of the applied gradient magnetic field and a velocity of a spin. That is, a phase of a spin can be expressed as a function of an intensity and an application period of a gradient magnetic field and a velocity of spins. Therefore, a blood flow velocity and a direction of blood flow can be calculated from phase information of spins.
As another non-contrast-enhanced MRA technology, a TOF (time of flight) method is known. The TOF method is a method to apply a saturation pulse to a blood flow to be a target and to image saturated blood signals flowing into an imaging section using inflow effect. In the PC MRA method and the TOF method, a blood vessel image is obtained as a longitudinal relaxation (T1) weighted image with a sequence of FE (field echo) type.
However, in the conventional MRA with ECG synchronization synchronous, an appropriate delay time is determined by performing an ECG-prep scan or another method before performing an imaging scan. Therefore, in case where a state of an object when a delay time is determined is different from that in performing an imaging scan, an appropriate delay time may be also changed. When an appropriate delay time during an ECG-prep scan is different from that during an imaging scan, an imaging scan is performed with an inappropriate delay time determined by performing the ECG-prep scan. Consequently, there is a problem that data is acquired at inappropriate timings and a blood vessel image cannot be obtained with a steady image quality and contrast.